Thermoplastic resin-coated aluminum alloy plate and method and apparatus for manufacturing the same

ABSTRACT

The present invention aims to offer thermoplastic resin coated aluminum alloy sheet having excellent adhesion after forming, wherein the laminated thermoplastic resin layer will not peel off even when subjected to such severe forming as drawing, followed by stretch-forming, further followed by ironing. In order to achieve this, aluminum alloy sheet is treated with alkali solution by dipping into or spraying alkali solution, rinsed, treated with acid solution by dipping into or spraying acid solution so as to obtain a specific surface condition, and further laminated with thermoplastic resin with or without adhesive on both sides of the aluminum alloy sheet.

TECHNOLOGICAL FIELD

[0001] The present invention relates to a thermoplastic resin coatedaluminum alloy sheet, suitable for use in which a severe forming isgiven, manufacturing method thereof, and manufacturing device thereof.More specifically, it relates to thermoplastic resin coated aluminumalloy sheet for use in can which is required to have severe formabilityand formability adhesion, intended to be formed into not only can lid ordrawn can, but also for drawn and ironed can, drawn and stretch-formedcan, drawn, stretched and ironed can and such, manufacturing methodthereof and manufacturing device thereof.

BACKGROUND TECHNOLOGY

[0002] Aluminum alloy sheet which is laminated with thermoplastic resin,for instance, polyester resin, is already being used for can lid andsuch. However, if the adhesion of the laminated thermoplastic resinlayer to the aluminum alloy sheet is insufficient, the thermoplasticresin layer might peel off during the forming, or corrosion mightprogress from the part where adhesion is insufficient. This adhesionafter forming is greatly influenced by the surface condition of thealuminum alloy sheet in addition to the formability of the aluminumalloy sheet and the characteristics of the laminated thermoplastic resinlayer. Thus, in order to improve the adhesion of the thermoplastic resinlayer or the coating film to the aluminum alloy sheet, the followingsurface treatments are conventionally given to the aluminum alloy sheet.

[0003] (1) Method of giving chemical treatment such as phosphorus acidtreatment or chromic acid treatment to the aluminum alloy sheet.

[0004] (2) Method of coating thermosetting resin primer on one side ofthe thermoplastic resin film or aluminum alloy sheet.

[0005] (3) Method of forming an anodic oxide film with minute pores,having diameter of 200 angstrom or more and pore depth of 5 μm or less,on the surface of the aluminum alloy sheet, using a solution containingchromic acid (Japanese laid open publication Hei 3-44496).

[0006] (4) Method of forming an oxide film of 20 angstrom or more on thealuminum alloy sheet by heating said sheet in atmosphere for two hoursor more in temperature range of 250-650° C. after said aluminum alloysheet is cleaned. (Japanese laid open publication Hei 6-272015)

[0007] (5) Method wherein after the aluminum alloy sheet is cleaned, itis electrolytically treated using oscillating current in an alkalisolution and an oxide film, having a thickness of 500 to 5000 angstromand ramified micro pores is formed on the aluminum alloy sheet (Japaneselaid open publication Hei 6-267638).

[0008] In the method of the chemical treatment such as phosphorus acidtreatment or the chromic acid treatment of method (1) phosphate,chromate, or fluorine compound, and such, are mainly used for thetreating solution. The thus formed chemical treatment film is effectivein the improvement of adhesion and generally used. However, enormousfacility for waste water treatment is needed for draining of thetreating solution in order to prevent environmental pollution.Therefore, such chemical treatments are undesirable from the viewpointof environmental protection. The method of coating a primer for adhesionof method (2) increase the cost by coating the primer and requires asurplus process for baking the coating as well, and is not desirablefrom the viewpoint of productivity. Furthermore, it needs an exhaustprocessing equipment of the organic solvent. As for the method offorming the anodic oxide film having pores of a specific diameter anddepth by the use of chromic acid solution of method (3), it is notdesirable from the viewpoint of productivity since the anodic oxide filmrequires a long time to be formed. Also the thermoplastic resin layersmight peel off when subjected to severe forming processing. Moreover, itrequires a facility for waste water treatment for environmentalpollution prevention. As for method (4), in which the aluminum alloysheet is heated for a long time in the atmosphere to form an oxidefilmed thereon, the thermoplastic resin layer peels off when subjectedto severe forming, like Method (3). If further requires a long time toform the oxide film and is not desirable from the viewpoint ofproductivity. The method (5), in which an oxide film of 500 to 5000angstrom is formed by electrolytical treatment using oscillating currentin the alkali solution, enables continuous surface treatment with shortperiod of time by electrolysis and is effective in the adhesion of thelaminated resin film. However, after the resin film is laminated, saidlaminated resin film peels off when subjected to such severe forming asdrawing, followed by stretch-forming and further ironing. Therefore, theadhesion after forming is far from sufficient and cannot endure severeforming.

DISCLOSURE OF THE INVENTION

[0009] The technical object which the present invention aims to achieveis to provide a thermoplastic resin coated aluminum alloy sheet, ofwhich the thermoplastic resin layer has excellent adhesion after formingand does not peel off even in further severe forming, compared to theabove-mentioned, conventionally surface-treated aluminum alloy sheet,the manufacturing method and the manufacturing device thereof.Concretely, it is to provide a thermoplastic resin coated aluminum alloysheet having excellent adhesion after forming, in which thethermoplastic resin layer dose not peel off ever when subjected to deepdrawing followed by stretch-forming and further followed by ironing andto provide a manufacturing method thereof and manufacturing devicethereof which are cost-effective, environment-friendly and enable rapidproduction.

[0010] The thermoplastic resin coated aluminum alloy sheet of thepresent invention is characterized in that at least one side of analuminum alloy sheet having comparative surface area increase rate of3-30% is covered with thermoplastic resin.

[0011] Said aluminum alloy sheet has minute pores formed on the surface.The minute pores have preferably an average diameter of 50 to 3000 nm,maximum depth of 1000 nm or less, and the duty area rate of 10 to 90%.Moreover, it is more preferable that the average diameter is 200 to 900nm and the depth is shallower than half of the diameter and that thepores are formed in the thickness direction of the aluminum alloysurface.

[0012] Moreover, it is preferable that the thermoplastic resin ispolyethylene terephthalate, copolyester resin mainly composed ofethylene terephthalate unit, polyester resin mainly composed of butyleneterephthalate unit and compound resin of polyester resins blended and/ormulti-layered.

[0013] In addition, it is preferable that the polyester resin layer tobe laminated is a multi-layered resin comprising an upper layer and alower layer of polyester resin, and an intermediate layer of blendedresin in which bis-phenol A carbonate is blended to polyester resin orbisphenol A polycarbonate.

[0014] The method of manufacturing the thermoplastic resin coatedaluminum alloy sheet of the present invention is characterized in thatan aluminum alloy strip is continuously treated in an alkaline aqueoussolution, rinsed with water, treated in acid aqueous solution, rinsedwith water, dried, and after that covered with thermoplastic resin.

[0015] It is preferable that the alkali aqueous solution is an aqueoussolution having as the main component of one or more than one compoundselected from the group of hydroxide, hydroxide, carbonate, bicarbonate,silicate, and borate of alkali metal or ammonium in quantity of 10 to200 gm/l.

[0016] Moreover, the above-mentioned acid solution preferably has as themain component 10 to 300 g/l of one or more than two types selected fromsulfuric acid, nitric acid, hydrochloric acid, and phosphoric acid.

[0017] In addition, it is preferable that the treatment with alkaliaqueous solution is spraying treatment or the dipping treatment in thealkali aqueous solution, and that the treatment with the acid aqueoussolution is spraying treatment or dipping treatment in the acid aqueoussolution.

[0018] The manufacturing device of the thermoplastic resin coatedaluminum alloy sheet of the present invention is characterized in thattanks for the alkali aqueous solution treatment, rinsing, acid aqueoussolution treatment, rinsing, a drying device, and laminating device forthe thermoplastic resin are serially placed next to each other.

THE BEST MODE OF CARRYING OUT THE INVENTION

[0019] An aluminum alloy sheet is dipped in the alkali aqueous solutionsuch as sodium hydroxide or the alkali aqueous solution is sprayed.After rinsing, said sheet is dipped in acid aqueous solution such assulfuric acid or the acid aqueous solution is sprayed, and thus thesurface of the aluminum alloy sheet is adjusted to a specific condition.After, it is rinsed, dried, and heated to a temperature higher than themelting point of thermoplastic resin to be laminated, thermoplasticresin is laminated on both sides of it by heat bonding. A thermoplasticresin coated aluminum alloy sheet having excellent adhesion afterforming, of which the laminated thermoplastic resin layer will not peeloff even after severe forming such as deep-drawing followed bystretch-forming, further followed by ironing, is obtained by a simpleand inexpensive method as mentioned above.

[0020] The present invention is explained further in detail as follows:First of all, the aluminum alloy sheet to be used in the presentinvention is not especially limited as long as it can endure severeforming such as deep-drawing, followed by stretch-forming, furtherfollowed by ironing, which is the object of the present invention.However, from the viewpoint of cost and formability, aluminum alloysheet of JIS 3000 series and JIS 5000 series with thickness of 0.20 to0.35 mm, which is widely used for cans, is preferable. Since thealuminum alloy sheet to be used in the present invention is formed afterthermoplastic resin is laminated, it is not necessary to consider thesolid lubrication of the surface, as is the case in drawn and ironedcan. The sheet may be selected considering formability, etchability, oradhesion after forming with the thermoplastic resin to be laminated.

[0021] Next, the surface condition of the aluminum alloy sheet isexplained. The surface condition formed on the surface of the aluminumalloy sheet of the present invitation by alkali aqueous solutiontreatment, followed by acid aqueous solution treatment, may be specifiedby the measurement with an atomic force microscope, for instance.Concretely, five arbitrary points on the surface of the aluminum alloysheet, treated by the alkali aqueous solution treatment followed by acidaqueous solution treatment, are measured. If the specific area increaserate is in the range of 3 to 30%, more preferably 4 to 20%, the objectof the present invention, an aluminum alloy sheet having excellentadhesion after forming, that is the object of the present invention,wherein the laminated thermoplastic resin layer will not peel off evenwhen subjected to severe forming, can be obtained. It is especiallypreferable that the average diameter of the formed minute pores is inthe range of 50 nm to 3000 nm and the maximum depth is 1000 nm or less,and the duty area rate of the pores is in the range of 10 to 80%. It iseven more preferable that the average diameter on the pores is in therange of 50 to 1000 nm, the maximum depth is 600 nm or less, and theoccupation area rate of the duty area rate of the pores is 20-80%.

[0022] In the adhesion after forming of the laminated thermoplasticresin layer the with the aluminum alloy sheet, the surface condition ofthe aluminum alloy sheet is very important. The conventional method forsurface roughening generally performed such as mechanical roughening orelectrolytic etching is effective in the adhesion after forming of thethermoplastic resin layer to be laminated. However, laminatedthermoplastic resin layer peels off when severe forming, which is thepurpose of the present invention, is given. Although the cause of thispeel-off is not clearly known, it is supposed that the thermally meltedthermoplastic resin dose not sufficiently penetrate into the dentedportions of the roughness of the aluminum alloy sheet, that is, theanchor effect is not enough.

[0023] In the present invention, it is supposed that when the specificsurface area increase rate is in the range of 3 to 30%, on the surfaceof the aluminum alloy sheet, and it has duty area rate of pores of 10 to80%, with minute pores of which the average diameter is 50-3000 nm andthe maximum depth of 1000 nm or less, and more preferably specificsurface area increase rate by 3 to 30%, duty area rate of pores of20-80%, average diameter of 50-1000 nm, and maximum depth of 600 nm orless, the thermally melted thermoplastic resin sufficiently penetratesinto the dented portions formed on the surface of the aluminum alloysheet and enough anchor effect is obtained. That is, it has been shownthat as the surface condition of the aluminum alloy sheet which comesinto direct contact with the thermoplastic resin layer, one withspecific surface area has excellent adhesion with the laminatedthermoplastic resin layer.

[0024] The surface area in this context differs from the conventionalconcept of the surface roughness and such measured by the stylusexamination method. It is similar to the concept of the so-calledsurface activation degree, or the surface area of a surface conditionwhere ultra minute ruggedness of nanometer order is formed.

[0025] The specific surface area increase rate mentioned in the claimsof the present invention was given as follows: the ratio (specificsurface area) of the area measured in a sample of the present invention(real area) to that of the sample tentatively having no ruggedness(projected area) was measured and the increment was expressed bypercentage. As for the actual measurement, an area of 5 μm square of thesurface of the sample was measured with 512 pixels (the number ofpixels) per line by an atomic force microscope “Nano scope IIIa”manufactured by Digital Instruments Inc,. The measurement was practicedon 5 different visual fields. The average was defined as the actualsurface area (numerator) A. The projected area (denominator=standard) Bof the measured field which was assumed to be entirely flat was alsodetermined. The increment of the ratio of A to B was calculated. Afterthat, specific surface area increase rate C was defined as C%=(A/B−1)×100.

[0026] In the present invention, it was found that when the thusmeasured specific surface area increase rate of the aluminum alloy sheetwas in the range of 3-30%, preferably in the range of 4-20%, theadhesion after forming with the laminated thermoplastic resin layer wasremarkably improved and the excellent adhesion after forming whichendures severe forming could be obtained. When the specific surface areaincrease rate is 3% or less, almost no effect is observed on theadhesion after forming, whereas when it is 30% or more, the maximumdepth of the formed minute pores becomes remarkably deep, which causesundesired effects on the adhesion after forming. Therefore, it is notpreferable.

[0027] It was mentioned earlier that in the present invention, it ispreferable to define the above mentioned specific surface area increaserate as well as the average diameter and the maximum depth of the formedminute pores and its duty area rate into the specified range. The reasonfor this definition is the same as that in the case of specific surfacearea increase rate. That is, when the average diameter of the pore is 50nm or less and the duty area rate of pores is 10% or less, it dose notaffect the adhesion after forming of the thermoplastic resin layer,whereas when the average diameter exceeds 3000 nm, the maximum depthexceeds 1000 nm and the duty area rate of ores exceeds 80%, the surfacebecomes too rough, even in the microscopic observation. It causes aninsufficient anchoring effect, which has fear for decreasing theadhesion after forming, since the melted thermoplastic resin does notsufficiently penetrate into the dented portions. It is not preferable.

[0028] Next, the method of manufacturing the thermoplastic resin coatedaluminum alloy sheet of the present invention is explained. First,aqueous solution mainly composed of one or more than one kind ofcompound of hydroxide, carbonate, bicarbonate, phosphate, silicate orborate of alkali metal or ammonium, or these alkali solution containingsurface active agent is used for the treatment by alkali aqueoussolution. The main purposes for treating with the alkali aqueoussolution are to melt and remove the oxide film formed on the surface ofthe aluminum alloy sheet and to remove the oil adhered to the surface ofthe aluminum alloy sheet. The surface might be occasionally somewhatetched. The addition of the surface active agent is preferably done toimprove the wettability and the degreasing ability of the aluminum alloysheet with the alkali aqueous solution. As for the concentration of thecompound used, the range of 10-200 g/l is preferable, and the range of30-100 g/l is more preferable. The temperature of the alkali aqueoussolution is preferably in the range of 30-80 ° C., and more preferablyin the range of 45-60 ° C. As for the treatment method, the aluminumalloy sheet is dipped in the alkali aqueous solution or in an alkaliaqueous solution with the surface active agent added or it is sprayedwith this alkali aqueous solution. A short duration of treatment of 1-30seconds is enough, and the range of 3-15 seconds is more preferable.Although direct current electrolysis or alternating current electrolysisin the alkali aqueous solution may be thought of, these methods requireelectrolysis facility, and it is not favorable from the viewpoint ofcost. When the concentration of the alkali compound is 10 g/l or less,or the temperature of the alkali aqueous solution is below 30° C., ittakes a long time to remove the adhered oil and the oxide film on thesurface of the aluminum alloy sheet, hindering the continuousproductivity of the thermoplastic resin coated aluminum alloy sheet ofthe present invention, and it is not preferable. When the concentrationof the alkali compound exceeds 200 g/l and the temperature of the alkaliis above 80° C., it is not only undesirable from the viewpoint of cost,but it is uneconomical since dissolving of the aluminum alloy surface ispromoted although the oil and the oxide film existing on the aluminumalloy surface is easily removed, that is also unfavorable. Local etchingmay be caused at times, and the aluminum alloy sheet having the surfacecondition required for the present invention might not be obtained andit is undesirable. In general, in case of the treatment using an alkaliaqueous solution of high concentration at a high temperature, thetreatment is sufficiently achieved in a short duration, whereas in caseof the treatment using an alkali aqueous solution of low concentrationat a low temperature, it takes a long duration. In the presentinvention, the concentration and the temperature of the alkali aqueoussolution, and the duration of treatment time are properly selectedwithin a specified range.

[0029] Next, the treatment with acid aqueous solution given afterrinsing is explained. It is desirable for the treatment with the acidaqueous solution to use solution mainly composed of one or more than onetypes of acid selected from sulfuric acid, nitric acid, hydrochloricacid and phosphoric acid. Carboxylic acid and oxicarboxylic acid arealso available for the acid treatment of the present invention. However,not only are they unfavorable from the viewpoint of cost, but theirchemical oxygen demand (COD) are higher compared to that of theinorganic acid such as sulfuric acid and additional cost for the wastewater treatment is required, and so they are not desirable. The purposeof the acid aqueous solution treatment is to remove the smut remainingon the surface, caused by the alkali aqueous treatment and to obtain asurface condition of the aluminum alloy sheet required for the presentinvention, having the specific surface area increase rate of 3-30%,minute pores having an average diameter of 50-3000 nm and maximum depthof 1000 nm or less, and the duty area rate of pores of 10-80% at thesame time. As for the concentration of inorganic acid used, the range of10-300 g/l is desirable, and the range of 30-150 g/l is more desirable.The temperature of the acid aqueous solution is desirably in the rage of5-60° C., and more desirably in the range of 15-40° C. As the treatmentmethod, the aluminum alloy sheet treated with the alkali aqueoussolution is dipped in the acid aqueous solution or sprayed by the acidaqueous solution. As for the duration of the treatment, a short durationof 1-30 seconds is enough, and the range of 3-15 seconds is moredesirable. Although a longer duration of treatment will not hinder toobtain a surface condition in which the present invention ischaracterized, it is not suitable for the high speed, continuousproduction of the thermoplastic resin covered aluminum alloy sheet ofthe present invention. Although there is also a method of etching thesurface of the aluminum alloy sheet by the direct current electrolysisor alternating current electrolysis by using this acid aqueous solution,the surface is locally etched by such electrolytic treatment, and thedesired surface condition cannot be obtained. Also an electrolysisequipment is needed, and it is economically undesirable. When theconcentration of the inorganic acid is 10 g/l or less or the temperatureof the acid solution is below 5° C., it takes a long time to obtain adesired surface condition, resulting in the hindrance of the continuousproductivity of the thermoplastic resin coated aluminum sheet of thepresent invention and thus is not desirable. When the concentration ofthe inorganic acid exceeds 200 g/l, although it will not hinder toobtain the surface condition in which the present invention ischaracterized, the amount of the acid solution taken out by thecontinuous treatment increases and therefor is economically undesirable.Moreover, with the rise of the temperature of the acid aqueous solution,not only does economical loss by heating become greater, but also thecorrosivity of the facility by the generated mist increases, andtherefor it is undesirable.

[0030] An aluminum alloy sheet having the desired surface condition,obtained by successive treatment with alkali aqueous solution and acidaqueous solution as mentioned above, is rinsed and dried, and followedby being laminated by a thermoplastic resin. For the lamination of thethermoplastic resin, both well-known extrusion lamination of meltedresin and film lamination can be applied. Moreover, applying acombination of both methods is also possible. The film laminating methodis suitable for a high speed production while the extrusion laminatingmethod of melted resin is advantageous in cost. The selection of eithermethod of the two should be decided considering characteristics requiredfor its use and so on.

[0031] In the present invention, as the thermoplastic resin to belaminated on the aluminum alloy sheet, one resin selected frompolyethylene, polypropylene, polyester, polyamide, polycarbonate,polyvinylchloride, polyvinylidene chloride, and acrylic resin, acopolymer of more than one of them, or a compound resin, blended of morethan one of them may be used. Each of these thermoplastic resin hasdifferent characteristics such as heat resistance, corrosion resistance,formability, adhesiveness etc., and they should be selected depending onthe intended use. In particular, for use where severe formability isrequired, for instance a can which is drawn, stretched-formed andfurther ironed, the following types of resins are preferable: polyesterresin, especially polyethylene terephthalate resin, copolyester resinmainly composed of ethylene terephthalate unit, polyester resin mainlycomposed of butylene terephthalate unit and a compound resin of theseresins blended. It is more preferable to use bi-axially oriented film ofthese resins. Furthermore, when impact resistance is required, thefollowing resins are desirable: a compound resin blended of the abovementioned polyester resin and bisphenol A polycarbonate resin, amulti-layered resin having the above-mentioned polyester resin as theupper and lower layer and compound resin blended of the above-mentionedpolyester resin and bisphenol A or bisphenol A polycarbonate resin asthe intermediate layer.

[0032] Moreover, in cases where the adhesion of said thermoplastic resinto the aluminum alloy sheet is not sufficient, or a layer ofthermoplastic layer alone cannot secure an enough corrosion resistance,thermosetting adhesive, for instance phenol epoxy adhesive on thesurface of the aluminum alloy sheet, after that the thermoplastic resinis laminated, or a method of applying the adhesive in advance on theside to be bonded of the thermoplastic resin film will be necessary.However, the method of using an adhesive leads to an increase in cost.Moreover, measures against environmental pollution caused by an organicsolvent contained in the adhesive are needed. Therefore, such adhesiveshould not be applied unless absolutely necessary.

[0033] The thickness of the thermoplastic resin layer to be laminatedshould also be defined considering the required characteristics. Ingeneral, the range of 5-50 μm is desirable and the range of 10-25 μm ismore desirable. The formation of a thermoplastic resin layer of 5 μm orless significantly lowers work efficiency in both the film laminatingmethod and the method of extrude-laminating of melted resin. It is alsoapt to generate pinholes and so sufficient corrosion resistance cannotbe obtained. On the other hand, the formation of a thermoplastic resinlayer of 50 μm or more is not economical compared to paints generallyused. Moreover, in case of necessity, additives such as stabilizer,antioxidant, anti-static additives, pigments, lubricants, and corrosioninhibitor can be added to these thermoplastic resins without causing anyobstruction.

[0034] The present invention is explained more in detail referring toexamples and comparative examples as follows.

EXAMPLE Example 1-6 and Comparative Example 1-4

[0035] As example 1-6 of the present invention and comparative example1-4, aluminum alloy sheets (JIS 3004H19) having sheet thickness of 0.26mm were subjected to the surface treatment under various conditions asshown in table 1, then rinsed and dried. Five arbitrary points of thethus surface treated aluminum alloy sheet were chosen, and the surfacecondition, that is, the average diameter of the pores, the maximumdepth, the duty area rate of pores, and the surface area increase rate,were measured with an atomic force microscope, and the average value wasobtained. The aluminum alloy sheets were heated to 240 ° C., and bothsides of them were simultaneously laminated with bi-axially orientedcopolyester resin film (thickness of 25 μm for the surface to be theinner surface of a can; thickness of 15 μm for the surface to be theouter surface of a can) consisting of 88 mole % of polyethyleneterephthalate, 12 mole % of polyethylene iso-phthalate, and then thelaminates were immediately dipped in water and quenched. After drying,approximately 50 mg/m² of paraffine wax was applied on both sides ofthem, and the laminates were subjected to the following forming.

[0036] First, after being punched out into a blank having a diameter of16 mm, it was formed into a drawn can having a diameter of 100 mm. Then,it was formed into a redrawn can having a diameter of 80 mm by redrawingprocessing. The redrawn can was subjected to stretching and ironingsimultaneously and was formed into a drawn and ironed can having a candiameter of 66 mm. This combined forming was performed according to thefollowing conditions: The clearance between the redrawing portion andthe ironing portion, that is to be the upper end portion of the can, is20 mm. The shoulder of the redrawing die is 1.5 times the sheetthickness. The clearance between the redrawing die and the punch is 1.0times the sheet thickness. The clearance in the ironing part is 50% offormer sheet thickness. Next, the upper end portion of the can wastrimmed off by a known method, and the neck-in processing and the flangeprocessing were given. The rupturing rate of the can wall and themetallic exposure of the inner surface of the can of the thus obtainedcan body, and the adhesion after forming of the aluminum alloy sheetwith the laminated thermoplastic resin layer were evaluated by thefollowing standards. The results are shown in Table 2. The metallicexposure of the inner surface of the can was determined by enamel rater(ERV value) measurement method. That is, 3% brine solution is pouredinto the obtained can, and a stainless stick is dipped as a cathode.With the can body as an anode, the voltage of about 6.3V is chargedbetween the two poles. At this time, if the aluminum alloy sheet underthe thermoplastic resin layer is exposed even slightly, a current flowsbetween the two poles. This current value was expressed as the ERVvalue, and the metallic exposure in the can was evaluated.

[0037] A) Rupturing rate of the can wall.

[0038] ⊚: 0%

[0039] ◯: Less than 10%

[0040] Δ: From 10 to less than 30%

[0041] ×: 30% or more

[0042] B) Metallic exposure of the inner surface of the can

[0043] *** (evaluated with enamel rater value [ERV=mA])

[0044] ⊚: 0 to 0.05 mA

[0045] ◯: 0.05 to 0.5 mA

[0046] Δ: 0.5 to 5.0 mA

[0047] ×: 5 mA or more

[0048] C) Adhesion after forming of the laminated resin layers(evaluated by the peeling off extent after the neck-in process)

[0049] ⊚: There is no peeling off at all.

[0050] ◯: Slightly peeled off, but there is no problem for practicaluse.

[0051] Δ: Considerably peeled off

[0052] ×: Peeled off at the entire upper end portion of the can

[0053] Incidentally, comparative example 1 of Table 2 was treated inalkali aqueous solution after which smut having poor adhesion wasremaining on the surface in the shape of cotton. Although the surfacewas significantly roughened, no minute pores are observed.

Example 7-12 and Comparative Example 5-8

[0054] As example 7-12 and comparative example 5-8, aluminum alloysheets (JIS 5052H39) having the sheet thickness of 0.26 mm weresubjected to the various surface treatments shown in Table 1, and afterthe treatments, the condition of the pores was measured as example 1 andsuch. These surface treated aluminum alloy sheets were heated to 235°C., and were laminated as follows: The surface to be the inner surfaceof the can was laminated with a double layered, bi-axially orientedpolyester film (thickness of 10 μm), comprising an upper layer ofcopolyester resin of 15 μm in thickness consisting of 88 mole % ofpolyethylene terephthalate and 12 mole % of polyethylene iso-phthalate,and a lower layer of polyester resin composed by blending 45 weight % ofcopolyester resin, consisting 94 mole % polyethlene terephthalate and 6mole % polyethylene iso-phthalate with 55 weight % of polybutyleneterephthalate. The surface to be the outer surface of a can waslaminated with the same bi-axially oriented film as example 1. Thesewere laminated simultaneously, and the laminates were dipped in waterand quenched. After drying, the laminates were formed under the samecondition as example 1 and such. The characteristics of the thusobtained can body were evaluated according to the same method as Example1 and such. The results are shown in Table 3.

[0055] Incidentally, comparative 5 or Table 3 was treated in alkaliaqueous solution after which smut having poor adhesion remained on thesurface in the shape of cotton. Although the surface is significantlyroughened, no minute pinholes are observed.

[0056] Industrial possibility

[0057] The thermoplastic resin coated aluminum alloy sheet of thepresent invention has excellent adhesion after forming, in which thelaminated thermoplastic layer dose not peel off, even when subjected tosevere forming. In addition, from the viewpoints of cost, environmentalpollution prevention and high speed productivity, it is possible to bemanufactured in an excellent method and therefore has extremely greatindustrial merit. TABLE 1 Surface treatment conditions of example 1-6and comparative Example 1-4 Treating conditions using alkali Treatingconditions using aqueous solution acid aqueous solution ConcentrationTemp. Duration Concentration Temp. Duration (g/l) (° C.) (sec.) (g/l) (°C.) (sec.) Example 1, 7 NaOH 15 60 15 H₂SO₄ 15 50 15 Example 2, 8 NaOH50 50 10 H₂SO₄ 70 15  5 Example 3, 9 Na₂CO₃ 100 80 3 HCl 50 25 10Example 4, 10 Na₂SiO₃ 60 45 10 HCl 20 45 10 Example 5, 11 Na₃PO₄ 30 4525 H₃PO₄ 150 30 25 Example 6, 12 KOH 150 45 5 HNO₃ 50 30  5 ComparativeEx. 1, 5 NaOH 50 50 20 Treatment omitted Comparative Ex. 2, 6 Treatmentomitted H₂SO₄ 70 15  5 Comparative Ex. 3, 7 NaOH 70 60 15 CrO₃ 50 65120  Comparative Ex. 4, 8 Na₂CO₃ 70 60 60 H₂SO₄ 100 65 60

[0058] TABLE 2 Surface condition and characteristics of Example 1-6 andcomparative Example 1-4 Condition of formed minute pores Duty area rateof Average Maximum pores (Specific Characteristics of Diameter depthsurface area can body (nm) (nm) increase rate) A B C Example 1 800 30035% (11.0%) ⊚ ⊚ ⊚ Example 2 400 200 50% (18.5%) ⊚ ⊚ ⊚ Example 3 800 40030% (7.0%) ◯ ◯ ◯ Example 4 1000  500 70% (25.0%) ◯ ◯ ◯ Example 5 2500 800 45% (15.0%) ⊚ ⊚ ◯ Example 6  90  90 12% (3.0%) ◯ ◯ ◯ Comparative —1200  — (32.0%) X X X Ex. 1 Comparative 300 150 7.5% (2.5%) ◯ Δ X Ex. 2Comparative 1000  1800  70% (25.0%) ◯ Δ Δ Ex. 3 Comparative 1800  1500 78% (35.0%) ◯ Δ Δ Ex. 4

[0059] TABLE 3 Surface condition and characteristics of Example 7-12 andcomparative Example 5-8 Condition of formed minute pores Duty area rateof Average Maximum pores (Specific Characteristics of Diameter depthsurface area can body (nm) (nm) increase rate) A B C Example 7 900 35044% (11.5%) ⊚ ⊚ ⊚ Example 8 400 230 55% (19.0%) ⊚ ⊚ ⊚ Example 9 13000 500 30% (9.5%) ◯ ◯ ⊚ Example 10 1600  600 78% (30.0%) ◯ ◯ ◯ Example 112300  750 50% (12.0%) ⊚ ⊚ ⊚ Example 12 150  80 12% (8.0%) ◯ ◯ ◯Comparative — 1300  — (35.5%) X X X Ex. 5 Comparative 550 450 9.5%(4.0%) ◯ Δ X Ex. 6 Comparative 1200  2000  70% (32.0%) Δ X Δ Ex. 7Comparative 2000  1800  75% (32.5%) ◯ Δ Δ Ex. 8

What is claimed is:
 1. A thermoplastic resin covered aluminum alloysheet, wherein a thermoplastic resin is laminated on at least one sideof an aluminum alloy sheet having specific surface increase rate of 3 to30%.
 2. The thermoplastic resin covered aluminum alloy sheet accordingto claim 1 , wherein minute pores are formed on said aluminum alloysheet, diameter of said minute pore is in the range of 50 to 3000 nm,the maximum depth of said minute pore is 1000 nm or less, and duty arearate of said minute pores is in the range of 10 to 90%.
 3. Thethermoplastic resin covered aluminum alloy sheet according to claim 2 ,wherein said minute pore is formed on the surface of said aluminum alloysheet on the conditions that average diameter of said minute pore is inthe range of 200 to 900 nm, the depth of said minute pore is shallowerthan half the diameter of said minute pore, and said minute pores areformed in the thickness direction from the surface of said aluminumalloy sheet.
 4. The thermoplastic resin covered aluminum alloy sheetaccording to any of claim 1 to 3, wherein said thermoplastic resin isselected from the group consisting of polyethylene terephthalate,copolyester resin mainly composed of ethylene terephthalate unit,copolyester resin mainly composed of butylene terephthalate unit, orcompound resin composed by being blended and/or multi-layered of thesepolyester resins.
 5. A thermoplastic resin covered aluminum alloy sheet,wherein a multi-layered resin, comprising an upper layer, a lower layerof polyester resin and an intermediate layer consisting of blended resinof polyester resin and bisphenol A polycarbonate or consisting ofbisphenol A polycarbonate, is laminated on the aluminum alloy sheetaccording to any of claim 1 to
 3. 6. A manufacturing method of athermoplastic resin covered aluminum alloy sheet, wherein an aluminumalloy strip is continuously treated in alkali aqueous solution, rinsed,treated in acid aqueous solution, rinsed and dried, after that,laminated with a thermoplastic resin.
 7. The manufacturing method of athermoplastic resin covered aluminum alloy sheet according to claim 6 ,wherein one or more than one compound mainly selected from the groupconsisting of hydroxide, carbonate, bicarbonate, phosphate, silicate andborate of alkali metal or ammonium is contained in said alkali aqueoussolution in quantity of 10 to 200 g/l, and one or more than one acidmainly selected from the group consisting of sulfuric acid, nitric acid,hydrochloric acid and phosphoric acid is contained in said acid aqueoussolution in quantity of 10 to 300 g/l.
 8. The manufacturing method of athermoplastic resin covered aluminum alloy sheet according to claim 6 or7 , wherein the treatment in said alkali aqueous solution is dippinginto said alkali aqueous solution or spraying said alkali aqueoussolution, and the treatment in said acid aqueous solution is dippinginto said acid aqueous solution or spraying said acid aqueous solution.9. A manufacturing device for a thermoplastic resin covered aluminumalloy sheet, wherein a treating tank for alkali aqueous solutiontreatment, a rinsing tank, a treating tank for acid aqueous solutiontreatment, a rinsing tank, a drying device and a device for laminating athermoplastic resin are connected in series.